PROJECT SUMMARY With the impending dementia epidemic, studies have attempted to demonstrate the potential to improve older adults' cognitive ability via cognitive training and activity interventions. However, results are inconsistent, and it is possible that potential benefits of an intervention may be missed due to a lack of sensitivity in the cog- nitive outcomes. Rather than implementing new interventions with modified techniques, there is a critical need to identify measures that are most sensitive to cognitive plasticity in current lifestyle interventions of aging. One promising approach is intraindividual variability (IIV) in cognitive speed, or moment-to-moment changes in a person's performance across multiple trials of a reaction time (RT) task. IIV is a stable individual characteristic that represents fluctuations in attention or executive control, and provides information beyond mean RT. IIV is maladaptive, as greater variability is associated with poorer cognitive performance, neurologi- cal disease, and poorer neurological functioning. IIV in cognitive speed in adulthood is hypothesized to be a sensitive behavioral indicator of neurological integrity, where greater IIV is indicative of compromised integrity, as has been shown to predict outcomes such as dementia and death. IIV is modifiable however, as short-term reductions in IIV have been found after attention-based feedback. Reduction in IIV via an intervention would demonstrate both cognitive and neurological plasticity. Overall, IIV is capable of detecting even modest changes in functioning that are not evaluated by typical cognitive tests, and represents a unique measure to evaluate cognitive change and provide information about underlying brain health in lifestyle interventions. The proposed project will examine if change in IIV occurred as a result of lifestyle interventions amongst older adults, and compare if there are differences in which intervention modalities elicit IIV change. Secondary data analyses will be conducted on datasets from three rigorous NIH-funded randomized control trials that asked older adults to participate in intensive physical, social, and cognitive activity. Participants in the treat- ment conditions are expected to show a reduction in IIV, representing an improvement in neurological function- ing, compared to the corresponding control groups. Investigation of intervention datasets that focused on differ- ent activity domains will also provide information regarding which activity domain(s) are critical to prompt cogni- tive gains. The resulting contribution will be significant because it will demonstrate the efficacy of using IIV to identify cognitive plasticity resulting from activity interventions with older adults, and provide insight into neurological plasticity without costly examination. Further, this project will have a positive impact in helping to identify activ- ity domains of particular importance for cognitive plasticity, which can then be used to enhance activity inter- ventions for older adults to improve cognitive functioning and potentially delay cognitive decline and dementia.